Documents about Germany’s Juelich nuclear research centre were found at Paris attacks suspect Salah Abdeslam’s Brussels flat, reports say.
The suspect, captured four days before the Brussels bombings, had internet print-outs and photos of the centre’s boss, media network RND said.
However, German intelligence officials have insisted the report is incorrect.
The Juelich research centre also put out a statement saying it had no indication of any threat.
The centre lies some 30km (20 miles) from the Belgian border and the German report suggested that material found printed out in the suspect’s flat in Molenbeek included photos of the research centre’s chairman, Wolfgang Marquardt.
RND cited sources within Germany’s parliamentary control committee saying they were briefed at the end of March by Hans-Georg Maassen, head of German domestic intelligence. An intelligence spokesman said later than Mr Maassen had never talked to any members of the committee.
Juelich’s reactor has been decommissioned and German officials insist all nuclear plants are safe from terror attack.
Salah Abdeslam, 26, was arrested after a four-month manhunt on suspicion of planning the 13 November Islamist gun and bomb attacks on Paris in which 130 people were killed.
His brother Brahim was among the bombers while he escaped back to Belgium and was eventually caught on 18 March at a friend’s home in Molenbeek, close to his own flat.
Four days later, Islamist associates of Salah Abdeslam bombed Brussels airport and Maelbeek metro station, killing 32 people.
Last month’s bombings have prompted a deepening political row in Belgium, after leaked EU reports from 2011 and 2015 highlighted flaws in security at the country’s airports.
Belgian federal transport agency chief Laurent Ledoux has resigned, complaining of a lack of funding from the Transport Minister, Jacqueline Galant.
He told Belgian radio station RTL that by stepping down he was doing what Ms Galant herself should have done – “take responsibility and step aside”.
Mr Ledoux said he had asked the minister in February for extra funding and staff to carry out security checks but she and her staff had refused to listen. However, he said it would be absurd to blame the minister for attacks.
The 2015 European Commission report, published by public broadcaster RTBF (in French), cited “serious deficiencies” and said airport security programmes, air carriers and suppliers were “not adequately monitored”.
Several Brussels bombing suspects were appearing in court on Thursday, days after they were arrested in a series of police raids.
Mohamed Abrini, identified by prosecutors as the third airport bomber, is said to have told police he deliberately failed to detonate his explosives. “I’ve never been to Syria, I wouldn’t harm a fly,” he said, according to Belgian daily SudPresse.
The abandoned bomb, prosecutors said at the time, exploded after the security forces had secured the scene and nobody was hurt, they added.
The suspect, 31, was caught on CCTV with Salah Abdeslam before the Paris attacks and his fingerprints were found in two “safe houses” in Brussels, according to prosecutors.
There was tight security outside the court in Brussels on Thursday where three other suspects, Osama Krayem, Bilal el-Makhoukhi and Herve BM, were also due to appear in a closed session.
Osama Krayem is suspected of buying the suitcases used to carry the Brussels bombs. He was also caught on CCTV with metro bomber Khalid el-Bakraoui shortly before he blew himself up.